Method of treating tungsten filaments



United States Patent METHOD OF TREATING TUNGSTEN FILAMENTS Thomas F.Smeaton, Rugby, England, assignor to General Electric Company, acorporation of New York No Drawing. Application November 6, 1953, SerialNo. 390,741

Claims priority, application Great Britain January 22, 1953 4 Claims.(Cl. 148-131) This invention relates to the manufacture of tungstenfilaments and more particularly to a method of treating tungsten wirewhich has been coiled into a single or multiple helix for use inelectric incandescent lamps or as electrodes for gas pressure dischargelamps.

The object of my invention is to produce tungsten filaments havingincreased efficiency and longer life than heretofore by treating thewire in such a manner that it is rendered more resistant to distortionand sag during the life of the lamp.

The method of manufacturing coiled tungsten filaments for use inelectric incandescent lamps is well known and usually consists in firstwinding the wire into a helix on a core wire and then winding the helixand its core onto a second core. In order to remove the strains causedby coiling, the filament is subjected to heat treatment in an atmosphereof hydrogen or hydrogen-containing gas. This heat treatment may becarried out while the tungsten filament is still on the mandrel Wire orafter the latter has been removed, quite frequently the treatment isgiven before and after the removal of the wire mandrel but the importantpoint is that with these heat treatments the tungsten filaments havehitherto been cooled in hydrogen or in an amosphere containing that gas.

I have found that the surface layer of tungsten wire, after it has beensubjected to heat treatment in a hydrogenous atmosphere and cooled inhydrogen, when exposed to air consists of oxide or contains adsorbedoxygen even when apparently clean and bright. It would appear that whenexposed to high temperature in hydrogen the outer layer is reduced andthe active metal so produced reverts to oxide on exposure to air;filaments which have gone through this cycle act as oxygen carriers, andas the oxygen is combined, getters are probably ineffective. Repeatedhydrogen furnacing and exposure to air results in a deepening of theoxide layer, the removal of which increases the amount of gas adsorbed,as would also any moisture content in the gas. The metal is penetratedby the atomic hydrogen probably mainly along the partially developedgrain boundaries but also by way of slip planes, surface inclusions oreven into dislocations. In this way a grain growth barrier is formed bythe molecular hydrogen, which traps the gas and develops suificientpressure to affect cohesion between grains. There is some evidence thatthe metal becomes contaminated by the running of dislocations carryingatoms of any contaminant surface into the metal. All such contingenciescause or induce sagging, even in the case of filaments made from metalwhich possesses the properties necessary for the rapid grain growthassociated with non-sag behavior.

In accordance with my invention, instead of allowing the filaments tocool in hydrogen or a hydrogen-containing atmosphere, the cooling takesplace in an atmosphere of inert gas such as argon after the filamentshave been subjected to heat treatment in a vacuum or in hydrogen or ahydrogen-containing gas.

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In carrying my invention into effect, the tungsten filament is heated inhydrogen for a period and at a temperature which is considered to beoptima, it is then cooled in an atmosphere of argon, nitrogen or othergas which is inert with respect to the tungsten filament. If the heattreatment takes place in a vacuum, the dry argon, nitrogen or otherinert gas can be admitted into the vessel in which the vacuum furnacingoperation is performed, thus breaking down the vacuum.

By way of specific example, a preferred method of practicing myinvention is to heat the tungsten filaments, either single orcoiled-coil, at 1500 C. for 10 to 15 minutes in a controlled atmosphereof wet hydrogen and immediately thereafter, without exposure to air, tocool the filaments to room temperature in dry argon in the case offilaments to be used in gas-filled lamps or dry nitrogen in the case of.filaments to be used in vacuum lamps. Best results are obtained byheat-treating the filaments in a vacuum at 1500 C. for about 10 to 15minutes and breaking the vacuum at the end of the heat treating cycle bythe admission of argon or nitrogen into the furnace.

Since subjecting coiled filaments to the process covered by thisinvention, I have found a remarkable improvement in the condition of thefilaments after a length of life greater than the generally acceptednormal life of a lamp. Moreover, the etficiency measured in terms oflumens per watt has been improved. It is believed that this improvementis due to the reduction or elimination of those chemical agents which,acting at grain boundaries and on the surface, hinder the grain growththus causing sag and low efliciency.

Although a preferred embodiment of my invention has been disclosed, itis recognized that variations and changes may be made therein within thespirit and scope of the invention as defined by the appended claims. Itis understood particularly that the heat-treating atmosphere, thecooling media and also the time and temperature can be varied,independently and in relation to each other, within fairly wide limitsto obtain the desired result.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of treating coiled tungsten wire filaments for electriclamps and the like said method comprising winding the wire into a helixon a mandrel wire, heat-treating the coiled filament to remove thestrain induced by the coiling operation in a controlled reducingatmosphere of hydrogen at about 1500 C. for about 10 to 15 minutes andimmediately thereafter, without exposure to air, cooling the filamentsin an atmosphere of nitrogen.

2. The method of treating coiled tungsten wire filaments for electriclamps and the like said method comprising winding the wire into a helixon a mandrel wire, heat-treating the coiled filament to remove thestrain induced by the ceiling operation in a controlled reducingatmosphere of hydrogen at about 1500 C. for about 10 to 15 minutes andimmediately thereafter, without exposure toair, cooling the filaments inan atmosphere of argon.

3. The method of treating coiled tungsten wire filaments for electriclamps and the like said method comprising winding the wire into a helixon a mandrel wire, heat-treatment of the coiled filaments to remove thestrain induced by the coiling operation in a vacuum at about 1500 C. forabout 10 to 15 minutes and immediately thereafter, Without exposure toair, cooling the filaments in an atmosphere of nitrogen.

4. The method of treating coiled tungsten wire filaments for electriclamps and the like said method Patented Oct. 8, 1957 "References (Jitedin the file of this patent UN TED STATES .EAI E TS Campbell Nov. 29,Ramage Jan. 19, Spaeth July 11, Mansfield Jan. 19,

1. THE METHOD OF TREATING COILED TUGSTEN WIRE FILAMENTS FOR ELECTRICLAMPS AND THE LIKE SAID METHOD COMPRISING WINDING THE WIRE INTO A HELIXON A MANDREL WIRE HEAT-TREATING THE COIL FILAMENT TO REMOVE THE STRAININDUCED BY THE COILING OPERATION IN A CONTROLLED REDUCING ATMOSPHERE OFHYDROGEN AT ABOUT 1500*C. FOR ABOUT 10 TO 15 MINUTES AND IMMEDIATELYTHEREAFTER, WITHOUT EXPOSURE TO AIR, COOLING THE FILAMENTS IN ANATMOSPHERE OF NITROGEN.